1. Field
The present application relates to an imaging device.
2. Description of the Related Art
In late years, video cameras and electronic cameras using a CMOS type imaging device become widely used. The CMOS type imaging device includes a plurality of pixels disposed in a two-dimensional array and having a photoelectric conversion part which converts a received light into an electric signal, and is made up of vertical signal lines for reading an electric signal output by each pixel, a vertical scanning circuit and a horizontal output circuit.
The CMOS type imaging device has an FD (floating diffusion) part to which a charge accumulated in the photoelectric conversion part is transferred and accumulated, and the charge transferred to the FD part is selected by a switch specifying an address of each pixel and is read to a vertical signal line on which a source-follower circuit is formed by a constant current generator. Signals read to the vertical signal lines simultaneously from pixels of one row are output from the imaging device by the horizontal output circuit.
In general, the CMOS type imaging device has larger noise as compared to a CCD type imaging device, and thus there have been made various proposals for reducing the noise.
For example, for removing noise occurring in each pixel, there is used a correlated double sampling circuit which reads an optical signal containing optical information and a reset signal containing a noise component from a pixel, and removing the noise component by subtracting the reset signal from the optical signal (see, for example, Japanese Unexamined Patent Application Publication No. H01-154678).
There is further considered a method to prevent signal saturation when high luminance is input by providing a clip circuit on an input side of a column amplifier, which forms a correlated double sampling circuit disposed on each vertical signal line (see, for example, Japanese Unexamined Patent Application Publication No. 2008-042675).
When a light with high luminance is input to the CMOS type imaging device, there may occur a problem that a signal charge of the photoelectric conversion part overflows into the FD part even when a charge transfer transistor of a photodiode PD is off. When the signal charge overflows into the FD part, it causes the level of the reset signal to fluctuate and become large, and thus the signal after subtracting the reset signal from the optical signal becomes small. For instance, as an extreme example, when the sun enters an imaging angle of view, a dark signal saturates and the difference from the optical signal becomes zero, which results in a problem that the portion of the sun in an output image becomes black.
Moreover, when the clip circuit is provided on the input side of the column amplifier, if an intense light at the level that does not cause the clip circuit to operate is incident before resetting the column amplifier, a signal overflowed into the FD part from the photodiode PD by the incident light appears on the output side of the column amplifier as a dark signal. As a result, the difference from the optical signal becomes small, which causes a problem that an output image of a pixel portion thereof turns to black or dark gray.
In general, the clip circuit which can be disposed in each column amplifier has a limitation on the accuracy of clip level. Thus, when the column amplifier is used with high gain, the above-described problems occur inevitably.
Particularly, an electronic camera taking a moving image or performing a live-view display not using a mechanical shutter, a light incident via an optical system cannot be blocked physically, and the photoelectric conversion part is constantly irradiated with a light. Thus, the problems as described above occur.
In view of the above-described problems, a proposal of the present application is to provide an imaging device which does not allow a high-luminance portion of an output image to be black when a high-luminance subject such as the sun is included in a moving image or a live-view display, and thus can achieve a high-quality live view and a moving image with high picture quality.